Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS₂ monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS₂ crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS₂ monolayers that appears at 2.28 eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. The microscopic origin is further corroborated by density functional theory calculations of dipole-allowed transitions in Fe:MoS₂. Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS₂ monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature.

This paper describes the fabrication and characterization of a hybrid nanostructure comprised of carbon nanotubes (CNTs) grown on graphene layers for supercapacitor applications. The entire nanostructure (CNTs and graphene) was fabricated via atmospheric pressure chemical vapor deposition (APCVD) and designed to minimize self-aggregation of the graphene and CNTs. Growth parameters of the CNTs were optimized by adjusting the gas flow rates of hydrogen and methane to control the simultaneous, competing reactions of carbon formation toward CNT growth and hydrogenation which suppresses CNT growth via hydrogen etching of carbon. Characterization of the supercapacitor performance of the CNT-graphene hybrid nanostructure indicated that the average measured capacitance of a fabricated graphene-CNT structure was 653.7 μF cm(-2) at 10 mV s(-1) with a standard rectangular cyclic voltammetry curve. Rapid charging-discharging characteristics (mV s(-1)) were exhibited with a capacitance of approximately 75% (490.3 μF cm(-2)). These experimental results indicate that this CNT-graphene structure has the potential towards three-dimensional (3D) graphene-CNT multi-stack structures for high-performance supercapacitors.

The optical and electronic properties of tungsten disulfide monolayers (WS₂) have been extensively studied in the last few years, yet growth techniques for WS₂ remain behind other transition metal dichalcogenides (TMDCs) such as MoS₂. Here we demonstrate chemical vapor deposition (CVD) growth of continuous monolayer WS₂ films on mm² scales and elucidate effects related to hydrogen (H₂) gas concentration during growth. WS₂ crystals were grown by reduction and sulfurization of WO₃ using H₂ gas and sulfur evaporated from solid sulfur powder. Several different growth formations (in-plane shapes) were observed depending on the concentration of H₂. Characterization using atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed etching of the SiO₂ substrate at low concentrations of H₂ and in the presence of an Ar carrier gas. We attribute this to insufficient reduction of WO₃ during growth. High H₂ concentrations resulted in etching of the grown WS₂ crystals after growth. The two dimensional X-ray diffraction (2D XRD) pattern demonstrates that the monolayer WS₂ was grown with the (004) plane normal to the substrate, showing that the WS₂ conforms to the growth substrate.